This invention relates generally to treatment of import grade liquefied natural gas (LNG) for uses in addition to formation and distribution of natural gas for commercial purpose. More specifically, it concerns such treatment to form engine fuel grade LNG, and/or to produce commercially distributable gas at elevated pressure, some of the gas used to drive an expansion turbine, to produce power.
Liquefied natural gas (LNG) is typically transported by ship to provide fuel in areas where there is insufficient indigenous natural gas. Once unloaded from the ship, it is stored in large storage tanks and then pumped and heated prior to being injected in gaseous state into a distribution pipeline. The primary end use for the natural gas is as fuel, where the exact chemical composition is of little concern.
There is however, an alternate use for LNG as a motor vehicle fuel where the LNG is carried on the vehicle in liquid form and, after conversion to warm gas, is combusted in an engine. Engines cannot tolerate many of the compounds frequently found in raw LNG, as they cause pre-ignition. High concentrations of many compounds, such as ethane, preclude normal LNG from being used as motor fuel.
It is possible to process LNG (heating grade) into LNG (vehicle engine fuel grade) by removing the undesirable compounds. See in this regard U.S. Pat. No. 6,986,266. One characteristic of conversion methods is the requirement for refrigeration. This raises both the capital cost and operating cost.